The present invention relates to a method and to an angle-determination apparatus for determining a rotor angle of a rotor of an electric machine. The invention further relates to an appropriate control apparatus.
In modern vehicles, internal-combustion engines are frequently being supplemented or replaced by electric motors. By way of electric drive machine in such electric and hybrid vehicles, a permanent-magnet synchronous machine (PMSM) is frequently employed nowadays, on account of its high efficiency and its small structural form or high power density.
The closed-loop control of the PMSM is ordinarily undertaken by a so-called field-oriented closed-loop control system. In this case, it is necessary to know the current absolute rotor angle of the PMSM at each moment with sufficient accuracy. The capture of this rotor angle is normally undertaken with a position encoder (such as, for example, a resolver, an incremental encoder, etc.). Alternatively, the rotor angle can be captured in encoder-free manner, in which case different methods may come into operation within various speed-ranges. In the medium and high speed-ranges, the so-called back EMF can be utilized. In this case, the fundamental-oscillation quantities of the stator voltages and stator currents are drawn upon as input variables for the method, and the rotor position is calculated therefrom without influencing the drive signals. For instance, U.S. Pat. No. 4,893,067 A presents a method in which the back EMF is utilized in order to determine the motor position.
At a standstill and in the low speed-range, the methods based on back EMF cannot be utilized, on account of the low voltage quantities. Rather, a method is needed that is able to ascertain the rotor angle independently of the speed.